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MANAGERS AND PROJECTIONISTS 35
NATIONAL ELECTRIC CODE.— A national code of rules based on the requirements of the fire underwriters. The requirements of the code must be observed as to inside electric wiring and other work in order to get insurance on a building. Copies of the -code may be had by applying to the National Board of Fire Underwriters, Electrical Department, Room 1100, No. 123 William Street, New York City. It is sent free of charge, but we would suggest that you send five cents in stamps for postage. It is a 216-page book.
NEGATIVE. — The opposite to positive. In electrical apparatus the pole toward which the current is presumed to flow.
NEGATIVE CARBON.— In a D. C. arc lamp the lower carbon to which the current flows across the arc from the positive carbon.
NEGATIVE FILM.— The film which is exposed to light in the camera. The film upon which the original image is impressed. The film from which positive prints are made.
NEGATIVE POLE.— Opposite pole to positive. In a dynamo or battery the pole to which the current is presumed to return from the external circuit.
NEGATIVE WIRE.— A wire attached to the negative pole. A wire having negative potential.
NEUTRAL WIRE.— In a 3-wire system the center wire or conductor is the "neutral.' It is negative to one outside wire and positive to the other, when either side is used separately. NEUTRAL WIRE AMMETER— An ammeter connected into the neutral wire to determine how nearly the load is in balance. It may be attached to any circuit or to the service wires of any building, if desired. The amount it records is the amount the load is out of balance.
NICKEL STEEL.— Ordinary soft steel to which a small percentage of nickel has been added. Best results are had with about 3.25 per cent, of nickel.
OBSERVATION PORT.— The opening in the front projection room wall through which the projectionist views the screen.
OHM. — The unit of resistance. See page 53. OHM'S LAW. — The law that, considering a uniform flow of current in a given circuit, the amperage is equal to the E. M. F., in volts, divided by the resistance in ohms. The law is expressed by simple formulas. See page 55.
OIL WELL. — (a) An oil-tight receptacle in which the intermittent movement of a modern projector is placed so that it